async signatures for kubeconfig loading is incorrect

[clux]

Current and expected behavior

As noted in #1645 and #1336

The kubeconfig file loading (which does need to do IO reading) is marked as async in the place it does not do any IO, and NOT marked as async in the place it does 🙃

Possible solution

Leave top-level fn as async, but swap around what is async;

kube/kube-client/src/config/file_loader.rs

Lines 28 to 36 in 3ee4ae5

	pub async fn new_from_options(options: &KubeConfigOptions) -> Result<Self, KubeconfigError> {
	let config = Kubeconfig::read()?;
	let loader = Self::load(
	config,
	options.context.as_ref(),
	options.cluster.as_ref(),
	options.user.as_ref(),
	)
	.await?;

read() should be async and probably use tokio::fs (unless people have better ideas) and load() should be sync (it's a pure fn). This means the top level-fns are still async and it's a relatively minor breaking change to only low-level config loader fns.

For people who do wish to have a purely sync opener function (perhaps because people usually block at the start of an app's init), we could potentially add a secondary Config loader that's not async (by bypassing async read), but ideally would like this to be an addition rather than a breaking change (as proposed and rejected in #1650)

Affected crates

kube-client